1. Field of the Invention
The invention relates to a display apparatus comprising an optocoupler circuit for transporting a signal between a non-line-voltage-insulated part and a line-voltage-insulated part of the display apparatus.
2. Description of the Related Art
Generally known display apparatuses, which comprise a switched-mode power supply to convert the AC line voltage into a stabilized power supply voltage, include an optocoupler between the non-line-voltage-insulated and the line-voltage-insulated part of the power supply. In this way, it is possible to transfer information about the value of the stabilized power supply voltage from the line-voltage-insulated part of the power supply to its non-line-voltage-insulated part to control on- and off-switching of a switch coupled to a primary winding of a transformer of the switched-mode power supply. The stabilized power supply voltage is supplied by a line-voltage-insulated secondary winding of the transformer. Line voltage insulation is required to prevent a hazardous electric shock when a user touches the display apparatus. An optocoupler comprises a light-emitting device and a photosensitive device which is optically coupled to the light-emitting device.
It is known to implement a low-cost non-line-voltage insulated power supply in a cathode ray tube display apparatus and to provide the line-voltage-insulation in the line output transformer (further referred to as LOT). The line deflection circuit is provided with power by the non-line-voltage-insulated power supply via a primary winding of the LOT. A line-voltage-insulated secondary winding of the LOT supplies a power supply voltage to signal processing circuits at the line-voltage-insulated part of the display apparatus. Signals originating from the line-voltage-insulated part and required to control or drive circuits at the non-line-voltage-insulated part cross the line voltage insulation barrier via pulse transformers or optocouplers. For each signal, which has to cross the line voltage insulation barrier, one pulse transformer, or one optocoupler, is used. Pulse transformers are unable to transfer DC signals, and are too bulky and expensive at low-frequent signals, like frame-frequent signals. Optocouplers are expensive.
It is, inter alia, an object of the invention to decrease a number of optocouplers required for transporting signals across the line voltage insulation barrier.
To this end, a first aspect of the invention provides a display apparatus comprising a non-line-voltage-insulated part; a line-voltage-insulated part; and an optocoupler circuit for transporting a signal between said non-line-voltage-insulated part and said line-voltage-insulated part, the optocoupler circuit comprising an optocoupler having a light-emitting device and a photosensitive device optically coupled to the light-emitting device, characterized in that the optocoupler circuit comprises a pulse generator for supplying a pulse signal having at least two different levels, at least one of said two different levels being controllable, to the light-emitting device; and a separation circuit coupled to the photosensitive device for separating an output pulse signal indicating an occurrence of a pulse, and a level signal corresponding to a value of said at least one controllable level.
The optocoupler circuit comprises a pulse generator for supplying a pulsed signal to the light-emitting device, the pulsed signal having at least two different levels of which at least one of the levels is controllable. The pulsed signal is a combination of an input pulse signal and an input level signal. The light-emitting device produces an amount of light which depends on the levels of the pulsed signal. The photosensitive device supplies an output signal which depends on the amount of light received from the light-emitting device. A separation circuit receives the output signal and separates an output pulse signal therefrom, indicating an occurrence of a pulse of the input pulse signal, and an output level signal corresponding to a value of the at least one controllable level, and thus to the input level signal. If the pulse signal has two different levels, the separation circuit may detect one or both of the transition(s), or one of the levels, to generate the output pulse signal. The separation circuit may detect the value of the controlled level, or may average the output signal if a single level is controlled. The separation circuit may detect the values of both controlled levels, or may average the output signal if both levels are controlled. It is possible to transfer more than two levels.
An embodiment of the invention in which the separation circuit comprises a comparator for comparing an output signal of the photosensitive device with a reference to supply the output pulse signal; and an integrator for integrating said output signal to supply the level signal, provides a simple implementation of the separation circuit.
An embodiment of the invention in which the deflection circuit comprises a frame deflection circuit, the input pulse signal being a frame synchronization signal, and the output pulse signal being supplied to the frame deflection circuit, provides a display apparatus in which the optocoupler transfers both the level signal and the frame synchronization pulses across the line voltage insulation barrier.
An embodiment of the invention in which the deflection circuit comprises a line deflection circuit, the input pulse signal being a line synchronization signal, and the output pulse signal being supplied to the line deflection circuit, provides a display apparatus in which the optocoupler transfers both the level signal and the line synchronization pulses across the line voltage barrier.
An embodiment of the invention in which the pulse generator further receives an input level signal from the signal-processing circuitto control the at least one controllable level to obtain the level signal, the input level signal being a controlled DC value or a waveform, provides a display apparatus in which the level signal is a DC level, for example, for controlling an amount of rotation of the information displayed on a display device of the display apparatus, or a waveform, for example, a geometry correction waveform to correct the line or frame deflection current, or to generate a dynamic focussing voltage. The waveform may be line or frame-periodic.
The display apparatus with the optocoupler circuit in accordance with the invention is able to transfer both a pulse signal and a level signal via a single optocoupler, and thus a number of optocouplers required for transporting signals across the line voltage insulation barrier decreases.
An embodiment of the invention in which the optocoupler circuit has a transfer characteristic form a current through the light-emitting device to a current through the photosensitive device, said transfer characteristic being temperature dependent, wherein said driver circuit comprises an impedance and a current source for supplying a current to a node, both said light-emitting device and said impedance being arranged between said node and a further node, said impedance having a value for decreasing said temperature dependence of said transfer characteristic, provides a temperature-stabilized optocoupler circuit, which decreases the temperature influence on the transfer of the level signal by the optocoupler.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.